1. Field of the Invention
The present invention relates to a backup battery charging circuit for charging a backup battery used for the backup of data in a memory, a microcomputer, or clock function.
2. Related Art of the Invention
With recent spread of portable equipment, various apparatuses comprising clock function and the like require a backup battery for maintaining the necessary function even when a main battery serving as a power supply is removed and even when the voltage of the main battery goes lower.
When the main battery is removed and when the voltage of the main battery goes lower, the function of a charging current supply for the backup battery is lost. Thus, the voltage of the backup battery goes lower owing to the current consumption by a device that uses the backup battery as the power supply. When the voltage of the backup battery goes down to a certain voltage or lower, data disappears in the memory, the microcomputer, or the clock function that use the backup battery as the power supply. In order to reduce the possibility of such data disappearance, the backup battery is charged as much as possible during normal operation.
A backup battery charging circuit according to the prior art has configuration as shown in FIG. 2. In FIG. 2, numeral 1 indicates a main battery (such as a secondary battery) serving as a power supply. Numeral 2 indicates a backward current protection diode. Numeral 3 indicates a regulator circuit. Numeral 4 indicates a limiting resistor. Numeral 5 indicates a backup battery.
The operation of this backup battery charging circuit is described below. The regulator circuit 3 receiving the power in the form of the electric potential obtained as the voltage VMB of the main battery 1 minus the forward voltage VT of the backward current protection diode 2 supplies a charging current to the backup battery 5. The backward current protection diode 2 prevents that a current leaks from the backup battery 5 to the main battery 1 when the voltage of the main battery 1 goes lower. The limiting resistor 4 avoids an excessive charging current.
Since the withstand voltage of the backup battery 5 is lower than the voltage VMB of the main battery 1, the output voltage VRG of the regulator circuit 3 is set at a voltage not exceeding the withstand voltage of the backup battery 5.
Nevertheless, when the voltage VMB of the main battery 1 goes lower, the output voltage VRG of the regulator circuit 3 goes lower as shown in FIG. 4. FIG. 4 shows the electric potential obtained as the main battery voltage VMB minus the forward voltage VT of the backward current protection diode 2, the voltage VBU of the backup battery 5, and the output voltage VRG of the regulator circuit 3. The interval (1) indicates a backup battery charging interval.
At the time when the output voltage VRG of the regulator circuit 3 goes lower than the voltage VBU of the backup battery 5, the charging becomes impossible. After that, the electric charge in the backup battery 5 is continuously consumed by the memory, the microcomputer, or the clock function. This causes a possibility of data disappearance unless the backup battery 5 is recharged.